1. Field of the Invention
The present invention relates to an image reading apparatus which reads an electrical signal of image information derived by scanning a plurality of photo-electric conversion elements relative to an optically coupled image. As an application, the present invention may be applied to an image scanner for a reflection type document sheet or a transmission type document sheet, or to an image reader unit of a copying machine or a facsimile machine.
2. Related Background Art
In an image reading apparatus which reads a color image, an image reading apparatus is known in which a color decomposition filter is inserted in an optical path between a document sheet image and a photoelectric conversion element to derive color-decomposed data of respective colors.
An example of such an image reading apparatus is disclosed in U.S. Pat. No. 4,953,01 assigned to the assignee of the present invention in which a plurality of photoelectric conversion elements (line sensors) are arranged at an appropriate pitch perpendicular (in a sub-scan direction) to a line direction (main scan direction), color decomposition filters are arranged for the respective line sensors, and reading of the respective color-decomposed data are assigned to the respective line sensors.
In this case, the line sensors sequentially output the read data from the pixels along the main scan direction in accordance with a synchronization signal, and the plurality of integral line sensors are physically moved relative to the document sheet image along the sub-scan direction to produce two-dimensional information.
In order to extract the color-decomposed data of the pixels on the document sheet image, it is necessary for each line sensor to correctly read the same pixel with a high positional precision. Considering one scan line on the document sheet image, the times when the parallelly arranged line sensors scan the scan line under consideration are different from each other but the line data of the respective line sensors are sampled in synchronism with the scan time of the scan line under consideration by the respective line sensors. Namely, the time required for the scan unit to move a distance between line sensors and the sampling period of the line sensors are synchronized.
In such an image reading apparatus, as the performance of the photo-electric conversion elements is improved, the read speed is increased and the amount of data to be processed is increased by the increased resolution and colorization. When a capability of a data processing unit connected to an output stage of the image reading apparatus is low and if nonintermittent image scan is continuously performed, the reading of the image data and the processing thereof are difficult to follow due to the image read operation because of the large volume of data and the high transfer rate. Thus, a so-called intermittent scan system is required in which the scan of a predetermined area of the document sheet image and the data processing by the succeeding stage data processing unit are intermittently repeated to complete the reading of the entire area of the image.
FIG. 7 shows a timing chart indicating the scan by the respective line sensors to the scan lines of the document sheet image when the image is read by the intermittent scan system in a prior art image reading apparatus.
In the prior art, a plurality of line sensors comprises a first line sensor and a second line sensor, and an interval therebetween is set to an 8-line length. An ordinate in FIG. 7 represents a relative scan position of-each line sensor to a scan line of the document sheet image, and an abscissa represents a time in the scan process. In the prior art, a stepping motor is used as a drive source for the sub-scan direction scan, and a sampling synchronization signal for extracting data by the line sensor for each scan and a drive synchronization signal for the stepping motor are shown.
The stepping motor is rotated in proportion to the drive synchronization signal, and the number of pulses of drive synchronization signal and the distance of scan drive of the line sensor have one-to-one correspondence. In the present example, the scan drive corresponding to a 3/8 line length is made at the fall of the drive synchronization signal (pulse signal). The sampling synchronization signal is used for the synchronization to sample the image data stored during the one-line drive time and transfer it to the succeeding stage signal processing circuit. In the present example, the image data is sampled simultaneously with the fall of the sampling synchronization signal.
In such an image reading apparatus, when 8 pulses of a motor drive synchronization signal are applied, the line sensors are driven by the 3-line length and the sampling synchronization signal is applied three times during that period so that three lines of image data are sampled, as shown in FIG. 7.
The stop and resume timing of the read scan in the intermittent scan mode depends on the capability of the succeeding stage data processing unit and an operation program, and inconvenience may occur if the stop and resume timing of the read scan is arbitrarily set in accordance with various data processing units to be connected in the succeeding stages.
For example, it is now assumed that the intermittent scan is performed four lines at a time in the prior art apparatus. In order to scan and drive the first sensor to the fourth line in a first scan, it is necessary to generate at least 11 pulses of a motor drive synchronization signal. After the 11-th pulse. Pmll (see FIG. 7) has fallen, a pulse Ps4 of the sampling synchronization signal falls and data in the fourth line is sampled. However, since the first line sensor is designed to be moved by 3/8 line length after the fall of the motor drive synchronization signal Pm11, it is now at a point S11 of FIG. 7.
Before the start of a second scan, the sampling sync signal and the motor sync signal are reset, and the second scan is started from the point S11 of FIG. 7. This includes an error ell (shown in FIG. 7) form an intended normal start of scan position. In a similar manner, an error e12 is involved at the start of the third scan, and an error e13 is involved at the start of the fourth scan, and the error gradually increases. As a result, the data of the-error image area generated after the completion of the respective scans are not extracted as the sampling data and the image data read by the first sensor is discrete for each scan and the image data in those areas are dropped.
Further, since the second line sensor is at a position spaced from the first line sensor by 8-line lengths, the area of the first scan line is partially read in the second scan. The second line sensor includes an error e21 which is equivalent to e12 at the start point of the third scan. As a result, the extracted data of the respective line sensors cannot be processed as one scan line data by merely imparting a delay between lines. Consequently, in the color image reading apparatus in which the reading of the color-decomposed data of the respective colors are assigned to the respective line sensors, the image data with color shift is supplied to the succeeding stage data processing unit.